Fluorescent flow cytometry allows the rapid characterization and purification of cells based on size and uptake of specific fluorescent probes. This technology has so greatly advanced the study of the cellular biology of immunological reactions that research in certain areas is now dependent on its availability. This proposal to obtain a fluorescent flow cytometer describes some of the projects of a group of investigators in the research laboratories of a university-based hospital having a long-term common interest in the pathobiologic mechanisms of rheumatologic and allergic diseases. These projects that would immediately benefit from the ready availability of a flow cytometer are: the genetic and acquired regulation of expression of cellular receptors for the C3b component in normal individuals and patients with rheumatologic diseases; the biochemistry and immunopharmacology of tissue mast cells; the cellular regulation of complement activation; the characterization of cell types having receptors for fibronectin and enumeration of these sites on macrophages in patients with compromised host defense; the identification of macrophage subpopulations having receptors for the lymphokine, migration inhibitory factor (MIF); the cellular regulation of the production of various forms of MIF; the immunoregulation of production of antibodies to DNA by lymphocytes from patients having systemic lupus erythematosus; the effects on lymphocyte subpopulations of total nodal irradiation in patients with rheumatoid arthritis; and the production of monoclonal antibodies having critical specificities.